Acids perform a wide variety of unique cleaning functions in both industrial and domestic settings. This includes removal of metal based oxides and scales such as rust and calcium carbonate. In addition, certain acids have been used to remove metal oxides and complex silicate based materials that leave a dull look to a surface. Some of these applications include, for example, atmospheric fallout removers in the automotive field, as well as the brightening of aluminum on truck trailers, and removal of road film in touchless vehicle wash.
Typically, in order to achieve effective results in a timely fashion in most of these applications, strong mineral acids such as hydrochloric or sulfuric acid are used, sometimes in conjunction with milder, weak acids such as citric, oxalic or glycolic acid. These acids can be useful in the removal of iron oxides and/or calcium carbonate scale, as well as being a first step in touchless vehicle washing. Other strong organic acids, such as alkane sulfonic acids, can also be useful but are less effective than mineral acids at rust removal and typically require fairly high use levels impacting cost.
In addition, concentrated cleaners containing these strong mineral acids are dangerous to use, often producing fumes that cause choking or more complicated and sometimes serious respiratory problems. Further, contact with skin can result in irritation of the skin to, in some cases, severe burning—depending upon the composition of the cleaner. The mechanism of the burns can involve acid catalysed hydrolysis of tissue (most common) and/or oxidative decomposition, depending upon the acid (e.g. nitric, sulfuric). As such, these complicating factors require the material to be classified as corrosive, which impacts the labeling of the product, the transportation mode available for the product, and therefore, at use cost.
Furthermore, in the case of the cleaning of aluminum truck trailers, a number of these acids have been employed to perform this function but with very limited success. There are two issues in cleaning these types of trucks, including both the removal of soils and road film, in addition to the removal of aluminum oxide which gives the metal a dull look. The clear leader in effecting both cleaning attributes, which is almost unique in the field, is hydrofluoric acid (HF). This acid is able to dissolve most any metal oxide, including aluminum oxide, as well as complex silicates. In the case of aluminum trailers, it leaves a very bright surface, almost white when used in conjunction with sulfuric acid.
Although HF has appeared to have unique properties, there are many issues regarding its use that make it undesirable. For example, HF can irreversibly etch glass which could render a windshield unusable due to opacification of the glass. In addition, HF can also “burn” the aluminum if left for too long on the surface. This phenomenon becomes more likely when cleaning is performed in hot weather. Burns result in a darkening (black or brown) of the surface. These can only be removed by polishing the metal, a time consuming and expensive solution. Furthermore, pitting of the surface can take place with overexposure. This is an irreversible phenomenon that will affect the appearance of the trailer thereafter.
Most notably, there are severe health and safety issues associated with the use of HF for the worker. Although it is a relatively weak acid with pKa in the region of 3.5, it is nonetheless extremely dangerous. HF will pass through the skin and can cause fluorosis, hypocalcemia and hypomagnesia. These are all very serious medical issues, and in severe cases, can lead to death. For example, death has been reported from spills of concentrated HF (48%) on as little as 2.5% of body surface area (relates to workers preparing cleaners). Even at low concentrations (less than 3%), HF exposure can lead to serious health issues (relates to workers in the field). One of the reasons that this can take place is the delayed action of HF in the body at these low concentrations. As it is not a strong acid, tissue is not damaged via a hydrolysis based mechanism which produces immediate pain. Symptoms may not appear for several hours, resulting in deeper penetration into the body and more severe burns requiring more invasive treatment strategies. In the US, there are more than 1,000 cases of medical emergency treatments due to HF exposure reported annually.
A further concern associated with long term exposure to HF is its propensity to bioaccumulate in human tissue. Even very low levels of HF that would produce no acute symptoms can cause serious health problems due to the very slow elimination from the body. The primary concern is embrittlement of the bones which has obvious, negative ramifications for the worker.
Attempts to minimize the dangers associated with HF have been made through the use of compounds such as ammonium bifluoride (ABF). This compound is the reaction product of one mole of ammonia and two moles of HF. The result is a solid material that is odourless, but nonetheless still very dangerous as there is still free HF associated with the product. Contact with the skin can still result in the same health issues found with HF (MSDS denotes a health rating of 4, extreme (life threatening), so it is really not a safe alternative.
Clearly, there remains a strong need to find a non-corrosive alternative for cleaning with HF. One technique that has been shown to minimize both health and safety as well as environmental problems associated with the use of general acid cleaning agents, has been the formation of “acid salts”. This involves the reaction of a strong mineral acid, such as hydrochloric acid, with a very weak base such as urea, which produce salts with extremely low pKa values, such that the salt still behaves as a strong acid. Accordingly, the salt formation creates a non-fuming product, and when a proper mole ratio is used, corrosivity of the skin can be greatly reduced. Urea hydrochloride, as more fully described in U.S. Pat. No. 5,672,279, is one such example. This compound however, although a safe and effective acid for descaling, suffers badly due to its aggressiveness to aluminum (burns the metal) and staining of metals such as stainless steel—severely limiting its use in transportation cleaning applications.
Other acid salts such as are formed from alkanesulfonic acids have been shown to be safe on aluminum and other metals such as stainless steel. However, they do not brighten aluminum in a timely fashion and typically require high use levels which impact costs significantly. This limits their use practically in at least this field.
Intuitively, one would postulate that formation of a urea HF salt would be an effective means to achieve improved safety with HF. However, given the relatively weak acid nature of HF (pKa 3.4), it is not possible to quantitatively produce a stable salt from the reaction of HF and urea. In essence, there will be some urea:HF salt formed in solution, but there will still be free HF in solution at any given time. A stronger base such as ammonia is required to quantitatively form a fluoride salt, however, the reaction product will be neutral and ineffective as a cleaner.
There remains a need to produce a safe product that will be effective as an HF-like cleaner. Such a product or composition should be effective at dissolving metal oxides (especially aluminum oxide) as well as complex silicates. Other favourable properties would include ability to rapidly solubilize iron oxides and calcium based scales. Mineral acids that can quantitatively form urea salts such as the halide series (HCl, HBr, HI), sulfuric and alkanesulfonic acids cannot meet these criteria, nor can the typical organic acids such as citric and glycolic acids. Nitric acid is, likewise, deficient, as a stable urea salt is unavailable due to oxidative degeneration.